FI124819B - Method and apparatus for maximizing energy efficiency for electric power systems - Google Patents

Description

METHOD AND APPARATUS FOR MAXIMIZING ENERGY EFFICIENCY OF AN ELECTRIC DRIVE SYSTEM

FIELD OF THE INVENTION

The present invention relates to electric drive systems, particularly to maximizing energy efficiency of an electric drive system.

BACKGROUND INFORMATION

Manufacturers Of Present-Day Frequency Converters Can Utilize Different Techniques In Controlling Behavior Of A Motor In A Respect To A Rotational Speed Of An Motor In An Electric Drive Application.

The applications can, for example, be divided into two groups on the basis of the behavior of the load: linear torque / speed ratio applications and quadratic torque / speed ratio applications. In linear (torque / speed ratio) applications, the torque applied to the load is directly proportional to the rotational speed. In quadratic (torque / speed ratio) applications, the torque is proportional to the square of the rotational speed.

Some linear applications such as constant-torque loads typically found in industrial applications may require high dynamic performance. In order to be able to maintain a full torque output from the motor at various motor speeds, the drive provides the motor with a nominal flux.

However, in some quadratic applications, such as pump or fan applications, dynamic performance requirements may not be demanding as in linear applications. In such applications, the flux is applicable by the drive can be limited, thus enabling more economic performance. On the other hand, this approach can result in a reduced dynamic performance of the drive, as there is more limited flux Capability available than with the nominal flux.

In some frequency converters, one of the above approaches, i.e. a more dynamic performance or a more economic performance may be selected as the default performance approach and the other may be selected by the user. The user does not, however, always select the more appropriate approach for the application in question.

BRIEF DISCLOSURE

An object of the present invention is to provide a method and an apparatus for implementing the method so as to alleviate the above disadvantages. The objects of the invention are achieved by a method and an arrangement which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The disclosed method allows automated selection of the operating mode, i.e. a dynamic performance mode where a nominal flux is used or an economic performance mode where the flux is limited in order to achieve energy savings.

The disclosed method first gathers a set of data points of torques at different rotational speeds. Then, the method calculates with which behavior of the load, i.e. the linear behavior or the quadratic behavior, the data points have better correlation. On the basis of the result of this calculation, one of the two torque / speed behaviors is selected to represent the torque characteristics of the system, and the motor is controlled on the basis of the selected behavior. As the behavior is automatically determined, selecting a more appropriate operating mode does not have to rely on user input.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which

Figure 1 illustrates linear and quadratic behavior of a torque in respect to rotational speed;

Figure 2 illustrates a flowchart of an exemplary implementation of the disclosed method; and

Figure 3 illustrates an apparatus for maximizing energy efficiency of an electric drive system comprising an electric motor and a load.

DETAILED DISCLOSURE

The present Disclosure discloses a method for maximizing energy efficiency of an electric drive system comprising an electric motor and a load. The disclosed method allows automatic detection of different behaviors of the motor in respect to the rotational speed of the motor, i.e. detection of different torque / speed ratios.

In the disclosed method, the torque characteristics of the system may be selected from two types of behavior: linear behavior and quadratic behavior of the torque in respect to rotational speed.

Figure 1 illustrates the linear and quadratic system behavior of a torque in respect to a rotational speed. The linear behavior (dotted line) represents the situation where the torque T is directly proportional to the rotational speed f

(1) where is a coefficient which represents the relation between the torque and the rotational speed, and b represents the constant torque which is independent of the rotational speed.

The quadratic behavior (dashed line) represents the situation where the torque is proportional to the square of the rotational speed f

(2)

Again, a represents the relation between the torque and the rotational speed, and b represents the constant torque which is independent of the rotational speed.

In order to be able to maximize energy efficiency, the revealed method automatically determines the torque characteristics of the system in question, i.e. which of Equations (1) and (2) better describes the system. The motor can then be controlled on the basis of the determined torque characteristics.

Determining torque characteristics may, for example, comprise first determining torque of the motor and rotational speed of the motor. The torque and rotational speed can be directly measured, or they can also be estimated, for example, by a frequency converter controlling the motor. If information on how much power is supplied to the motor is available, the torque can be calculated from the power. The rotational speed may be determined from the output frequency of the frequency converter.

The disclosed method can then gather the plurality of the data points where each data point represents the torque of the motor at the rotational speed of the motor.

On the basis of the data points, the value for the first parameter may be calculated. The first parameter represents how much the data points deviate from the quadratic behavior. The value for the second parameter is also calculated on the basis of the data points. The second parameter, in turn, represents how much the data points deviate from the linear behavior. Then, the first parameter may be compared with the second parameter, and the torque characteristics may be determined on the basis of the comparison.

When the torque characteristics have been determined, the motor may be controlled on the basis of the determined torque characteristics. On the basis of the torque characteristics, the operating mode, i.e. the dynamic performance mode where the flux is not limited or the economic performance mode where the flux is limited in order to improve energy efficiency can be chosen.

If the electric drive system is controlled on the basis of a torque reference, and if the electric drive can initially be set to economic performance mode, determining the torque characteristics can also be accomplished by monitoring the rate of change of the torque reference or the difference between the torque reference and the actual torque.

For example, the magnitude of the rate of change of the torque reference may first be determined. The magnitude can then be compared with the set limit, and if the magnitude exceeds the set limit, the electric drive can be set to dynamic performance mode.

Alternatively, the torque of the motor may first be determined. The difference between the torque reference and the determined torque may then be determined and compared with a set limit. If the difference exceeds the set limit, the electric drive is set to the dynamic performance mode.

Figure 2 illustrates a flowchart of an exemplary implementation of the disclosed method. In the first step 21 the torque and the rotational speed are measured and the data points are collected.

In the second step 22, the value for the first parameter is calculated. Two equations are formed on the basis of two data points, for example (/, /) and (/ 2, / 2). For each data point, the torque is represented by the square of the rotational speed multiplied by the first coefficient and incremented by the second coefficient. The equations for the two data points (/, /) and (/, / 2) have the same coefficients asq and bsq:

(3) (4)

The values of the coefficients asq and bsq may then be solved as follows:

(5) (6)

On basis of coefficients asq and bsq and rotational speed / 3 at third data point, an expected torque T3sq for rotational speed in third data may be calculated.

(7)

The difference between the expected torque T3sq and the torque T3 at the third data point may then be calculated, and the magnitude | J3 -T3sq \ of the difference may be used as the value of the first parameter.

In the third step 23, the value for the second parameter is calculated. Calculating the value for the second parameter in the third step 23 can be performed in a similar manner to the second step 22. The two equations are formed on the basis of two data points, for example {Thfi) and (Γ2, / 2 ). For both data points, the torque is represented by the rotational speed multiplied by a first coefficient and incremented by a second coefficient. The equations for the two data points (Γι, / i) and (Γ2, / 2) have the same coefficients aUn and bUn.

(8) (9)

Values of the coefficients aUn and bUn may be solved as follows:

(10) (11)

An expected torque T3! M for the rotational speed at the third data point is calculated on the basis of the coefficients aUn and bUn and for the rotational speedf3 at the third data point:

(12)

The difference between the expected torque Τ3Ηη and the torque T3 at the third data point may then be calculated, and the magnitude | J3 -T3Un \ of the difference may be used as the value of the second parameter.

In the fourth step 24 in Figure 1, the first parameter is compared with the second parameter and the torque characteristics of the system are determined on the basis of the comparison. The behavior, i.e. the linear or quadratic torque / speed ratio, which better fits the data points can be selected as the system torque characteristics.

Finally, in the fifth step, the motor is controlled on the basis of the determined torque characteristics.

Calculation of the first and second parameter is not, however, limited to the above examples. In some applications, where accurate measurements are not easily obtained, the method of least squares may, for example, be used. For example, the data points may be fitted to Equations (1) and (2) by using the method of least squares and the torque characteristics to be used may then be selected on the basis of the best fit. On the other hand, the method of least squares is computationally somewhat more complex than the three-point curve fitting as revealed in Equations (3) to (12).

Figure 3 illustrates an apparatus 31 for maximizing energy efficiency of an electric drive system comprising an electric motor 32 and a load 33. The load 33 in Figure 3 is a fan which is rotated by the motor 32. The apparatus 31 in Figure 3 is a frequency converter which controls the motor 32 and implements the disclosed method. The frequency converter 31 automatically selects the appropriate torque characteristics to be used depending on the application. The torque characteristics in the frequency converter 31 may be selected from two types of behavior: linear and quadratic behavior of the motor in torque respect to the rotational speed of the motor.

The frequency converter 31 acts as means for determining the torque of the motor and the rotational speed of the motor. In Figure 3, it has internal estimates of said variables. The frequency converter 31 gathers a plurality of data points to its internal memory. Each data point represents the torque of the motor at the rotational speed of the motor.

The frequency converter 31 in Figure 3 comprises computing means such as a microprocessor, a DSP, an FPGA, or an ASIC, which is used to calculate the value for the first and second parameter on the basis of the data points. The first parameter represents how much the data points deviate from the quadratic behavior of the torque, and the second parameter represents how much the data points deviate from the linear behavior of the torque. The calculation of the values for the first parameter and the second parameter can, for example, be performed as disclosed in the exemplary implementation of Figure 2.

After calculating the first and second parameter, computing means of the frequency converter 31 compare the first parameter with the second parameter, and determining the torque characteristics on the basis of the comparison. The frequency converter 31 then controls the motor by using determined torque characteristics.

The apparatus for maximizing energy efficiency can also be an external device attached to a frequency converter. The apparatus may determine the behavior of the system as disclosed above and may then set the frequency converter to an appropriate operating mode, i.e. the dynamic performance mode or the economic performance mode.

It will be obvious to a person skilled in the art that the Inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims (8)

A method for maximizing the energy efficiency of an electric drive system comprising an electric motor (32), a load (33) and a motor controlling frequency converter (31), characterized in that the method comprises the steps of using a frequency converter (31) to determine the torque characteristic of the system the characteristic curve is selected from two types of behavior: linear or quadratic torque behavior of the motor (32) with respect to the rotational speed of the motor (32), selecting an operating mode based on the specified torque characteristic and controlling the motor (32) in the selected operating mode. A method according to claim 1, characterized in that the step of determining the torque characteristic comprises the steps of determining the torque and speed of the motor (32), collecting a plurality of data points, wherein each data point represents the torque of the motor (32) based on the data points, where the first parameter describes how much the data points deviate from the quadratic behavior of the torque, calculating a value for the second parameter based on the data points, where the one parameter describes how much 3. A method according to claim 2, characterized in that calculating the value of the first parameter comprises the steps of generating two equations based on two data points, wherein the torque is the square of rotation speed for each data point multiplied by the first constant plus the second constant, solving the values of the constants, calculating the assumed torque for the third data point rotation speed based on the constant and the third point rotation speed, calculating the difference between the assumed torque and the third data point torque, using the difference of the first parameter whereby the torque is the rotational speed for each data point multiplied by the first constant and added by the second constant; and where both equations have the same constants, solving the values of the constants, calculating the assumed torque for the third data point at the third data point based on the constant and the third point rotation speed, calculating the difference between the assumed torque and the third data point torque. Method according to claim 2 or 3, characterized in that the torque is calculated from the power supplied to the specified motor (32). A method according to claim 1, characterized in that the electric drive system is torque controlled, wherein the electric drive is initially put into an economic performance mode wherein the leakage is limited to achieve energy savings, and wherein the torque determination comprises the steps of adjusting the torque reference magnitude to , and when the magnitude exceeds the set limit, the electric drive is set to a dynamic performance mode with no leakage limitation. The method of claim 1, wherein the electric drive system is torque controlled, wherein the electric drive is initially put into an economic performance mode wherein the leakage is limited to improve energy efficiency, and wherein the torque determination comprises the steps of determining engine torque, defining torque, comparing the difference with the set threshold, and if the magnitude exceeds the set threshold, the electric drive is set to a dynamic performance mode with no leakage limitation. Apparatus for maximizing energy efficiency of an electric drive system comprising an electric motor (32), a load (33) and a frequency converter (31) controlling the motor (32), characterized in that the device comprises means (31) for determining a system torque characteristic, the characteristic curve is selected from two types of behavior: linear or quadratic motor torque behavior with respect to the engine speed and means (31) for selecting a mode of operation based on the determined torque characteristic. Device according to claim 7, characterized in that the means (31) for determining the torque characteristic comprise means for determining the torque and rotation speed of the motor, means for collecting a plurality of data points, wherein each data point for calculating a motor torque at a means for calculating a value for the second parameter based on the data points, wherein the second parameter describes how much the data points deviate from the linear behavior of the torque, means for comparing the first parameter with the second parameter, of the specified torque in the control based on the female curve.